Since C axis is a little uncommon, I'm first going to ask what axis it is.
We have 1 milling machine with C axis (X Y Z B C). It is a rotary head machine, the head can turn 360degrees 27 times(9999.999degrees) before the software on the controller won't handle any more. Since it is positioned in degrees, the feedrate needs to be in 'inverse time'.
If yours is similar, do a google search for "inverse time feedrate calculation" (without the ""). Click the first page listed (actually a .pdf document). Scroll down to page 8. It gives an example with and formula(s).